[css-lists][css-counter-styles] Compute list-style-type to 'decimal' if the specified counter style does not exist

Hi,

Recently, there is a behavior change in Gecko which makes
list-style-type be computed to 'decimal' when the specified counter
style doesn't exist, rather than keeping the specified nonexistent one.
This change helps us save a string for common cases, and simplify some
code. None of which is a big deal, though.

I'm not completely sure what does the spec say for this case. It seems
to me the old behavior is more conformant, but I guess the new behavior
might be perferable, since it makes the computed value match the used
value. So I propose changing the spec to this behavior.

What do you think?

- Xidorn